Unwrapping curved Object
Select the left edge and press S, x, 0.
Repeat with the right edge.

This scales both edges to 0 in the horizontal direction, so they will become straight. They will be of different lenghts, so scale them up vertically to create a set of squares. (Look at this duplicate post for pictures)

One thing to mention: since in the UV editor the faces are rectangular and in the actual mesh they are trapezoids, the texture will result a bit distorted. It's not a problem if your camera is far away, but if it's close it will be quite ugly, and in that case i'm afraid the only way is to UV unwrap every single face in the belt in order to preserve their shape while being able to control their orientation (and since a conveyor belt is made of separated panels, I don't think that the seam between them would be so bad)